In April 1898 Rainer Maria Rilke, not yet twenty-three, began a diary of his Florence visit. It was to record, in the form of an imaginary dialogue with his mentor and then-lover, Lou Andreas-Salome, his firsthand experiences of early Renaissance art. The project quickly expanded to include not only thoughts on life, history, and artistic genius, but also unguarded moments of revulsion, self-doubt, and manic expectation. The result is an intimate...